Re: [dpdk-dev] [PATCH 0/4] support for write combining

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Apr 11, 2018 at 04:07:13PM +0200, Rafal Kozik wrote:
> Support for write combining.
> 
> Rafal Kozik (4):
>   igb_uio: add wc option
>   bus/pci: reference driver structure
>   eal: enable WC during resources mapping
>   net/ena: enable WC
> 
>  drivers/bus/pci/linux/pci_uio.c | 39 ++++++++++++++++++++++++++++-----------
>  drivers/bus/pci/pci_common.c    | 13 ++++++++-----
>  drivers/bus/pci/rte_bus_pci.h   |  2 ++
>  drivers/net/ena/ena_ethdev.c    |  3 ++-
>  kernel/linux/igb_uio/igb_uio.c  | 17 ++++++++++++++---
>  5 files changed, 54 insertions(+), 20 deletions(-)
> 
Couple of thoughts on this set.

You add an option to the kernel module to allow wc to be supported on a
device, but when we go to do PCI mapping, we either map the regular
resource file or the _wc one. Therefore:

1. Why not always have igb_uio support write-combining since it can be
controlled thereafter via userspace mapping one file or another?

2. Why not always map both resource and resource_wc files at the PCI level,
and make them available via different pointers to the driver? Then the
driver can choose at the per-access level which it wants to use. For
example, for init of a device, a driver may do all register access via
uncachable memory, and only use the write-combined support for
performance-critical parts. [I have a draft patch lying around here
somewhere that does something similar to that.]

One last question - if using vfio-pci kernel module, do the resource_wc
files present the bars as write-combined memory type, or are they
uncachable?

Regards,
/Bruce